The existing telephone network in the United States of America is largely a remnant of the earlier AT & T Network which was divested into several regional holding companies pursuant to an order of a Federal Court. This existing network had been designed to provide the equivalent of an electrical connection between two points in the network. The only information originating within the telephone network, and the conveyance of this information, was related to the provision of a basic capability to connect the two points.
Advances in the state of the art of communications have lead to the possibility of providing the existing services of the telephone network in a more efficient manner while simultaneously establishing the foundation for new network services beyond the capability of the existing network. The realization of this possibility has resulted in both national and international development of new standards for communication protocols between the switching elements within the telephone network. These new standards are widely referred to as "Signaling System #7 (SS7)". For example, see TR-NPL-000246 "Bell Communications Research Specification of Signaling System No. 7, Revision #2, 6/87."
These new services not only require common channel signaling on an inter-office basis, but have made apparent the requirement for common channel signaling to the end user customer or subscriber.
Common channel signaling uses a single dedicated control channel for all signaling functions related to a group of associated channels. The (pre-divestiture) U.S. Bell System installed its first common channel signaling facilities in 1976. A. E. Ritchie and J. Z. Menard, "Common Channel Interoffice Signaling, An Overview", Bell System Technical Journal, February 1978, pp. 221-236. These facilities are referred to as Common Channel Interoffice Signaling (CCIS) facilities.
The extant method of signaling from a terminal to the telephone central office switch to request service (e.g., circuit connection, operator service request, or circuit disconnection) or between central office and private branch exchange switches, is based upon in-band signaling techniques. Such systems transmit the signaling information in the same band of frequencies as that used by the voice or data signal. Frequently, existing data terminals use converters called modems (modulators/demodulators) to convert the data terminal signals into corresponding voice-frequency band signals. The most prevalent example of in-band signaling is single frequency (SF) signaling, which uses a 2600 Hertz tone as an on-hook (call termination) signal for inter-office trunks.
Two common examples of in-band signaling are those of push-button telephones which use dual-tone multifrequency (DTMF) signaling, and multifrequency (MF) signaling between switching offices.
As the majority of telephones in the United States of America are connected to crossbar offices, 1ESS (a Trademark of AT&T Technologies, Inc.) or 1/1AESS (a Trademark of AT&T Technologies, Inc.) switches, the replacement or upgrading of these switches would require vast economic expenditures by the telephone companies. Estimates of this replacement cost for a typical central offices are frequently in excess of $3 million. The incurring of such costs for the replacement of switching equipment which is functioning well is not justified by initial increases in revenues from the provision of more efficient switching.
The present invention provides a method and apparatus for implementing ISDN capabilities within Class 5 central offices such as the 1/1A ESS (a Trademark of AT&T Technologies, Inc.) analog SPC office. This is accomplished without affecting the underlying operating principles of the existing switching office. In addition, the ISDN capability is provided in a cost effective manner without adversely affecting the quality of service as perceived by the subscribers connected to such a switching office. Further, the ISDN capability is provided in such a manner so as not to adversely affect the operation of the upgraded switching office or the rest of the telephone system.
The Integrated Services Digital Network is an end-to-end digital network that supports a wide range of services accessed by a limited set of standard multi-purpose user-network digital interfaces.
ISDN provides a small number of standard interfaces, for example, the Basic Rate Interface (BRI) and the Primary Rate Interface (PRI), for high-speed digital voice and data services access, through a single interface to the user's premises. In particular, an ISDN access interface incorporates a common signaling channel to the end user, thereby extending the inter-office SS7 capability to the end user.
A number of ISDN trials have been taking place in Bell and independent telephone operating company networks, and limited commercial deployment of ISDN has begun in some metro areas. One of the main reasons for the slow progress of ISDN to date in the U.S. is the installed base of 1/1A ESS analog SPC offices, estimated to comprise about 54% of access lines in the U.S. in 1988, as well as 2/2B ESSs and 3ESSs. The ESSs provide valuable features and services now, and are not yet generally ready for replacement.
In addition is the base of electromechanical offices which are generally more suitable candidates for replacement by digital switching equipment.
These electromechanical and analog SPC switches are currently not capable of providing ISDN access or services.
Thus, heretofore implemented systems that provide ISDN capabilities on existing switches, be they electromechanical, analog, or even digital switches that do not have ISDN capability, generally requires either total switch replacement, or a non-integrated approach where an ISDN switch is co-located with the existing switch. The non-integrated approach results in the addition of a new switch, separate and distinct from the existing one.
In this "traditional" approach, the ISDN switch provides ISDN interfaces on the customer premises side, and analog line interfaces (such as analog loop-start or ground-start lines) as the interface to the existing switch for voice calls that enter the ISDN switch, while circuit-switched and packet data are switched directly in and out of the ISDN switch.
Several drawbacks or limitations to the heretofore implemented systems in providing ISDN capability are:
(a) One B-channel has to be dedicated for voice, and one B-channel for circuit switched or packet data on an ISDN BRI. This results in significantly less efficient port utilization on the ISDN BRI B-channels since efficiency mandates an exactly 50 percent split between those B-channels used for voice, and those that are used for data. PA1 (b) Separate telephone directory number (DNs) have to be used for voice and for data calls, the voice DN being a subset of the DNs of the existing analog switch, and the data DNs being part of the DNs of the ISDN switch.
This "traditional" approach for providing ISDN results in there being two separate switching entities, with very little interworking between them, i.e., a non-integrated approach.